A line driver circuit is a circuit that is used to place a differential voltage across two conductors, such as, for example, the conductors that make up a telephone line, the traces of a printed circuit (PC) board, or any other type of electrically conductive transmission medium. A typical line driver circuit steers current through an N field effect transistor (NFET)/P field effect transistor (PFET) device that acts as a current source to set the current in the transmission medium load and to set the output amplitude of the driver circuit.
FIG. 1A is a block diagram of a known line driver circuit 1 connected by a transmission medium to a receiver circuit 11. The driver circuit 1 includes first and second current sources 2 and 3 that mirror each other and that cause an output voltage differential to be produced across output terminals 4 and 5 of the driver circuit 1. The current sources typically are Field Effect Transistors (FETs) fabricated in a Complementary Metal Oxide Semiconductor (CMOS) process. An explicit transmitter_terminating resistor REX is connected between the output terminals 4 and 5 of the driver circuit 1. The receiver circuit 11 is connected to the output terminals 4 and 5 of the driver circuit by a transmission line comprising conductors 14 and 15, respectively. In a typical system, the impedance of the transmission line (RTL) is ideally matched by the receiver's termination resistor RLOAD.
The driver circuit 1 has switches 6-9 the are switched in a particular manner to control the polarity of the signal output from the driver circuit 1 at output terminals 4 and 5. The driver circuit operates as follows. When switches 6 and 8 are closed, the driver current follows the paths represented by arrows 12A and 12B. The portion of the current represented by arrow 12A passes through the explicit terminating resistor REX and continues along the path shown. The portion of the current represented by arrow 12B continues along the transmission line 14 to the receiver circuit 11 and through the receiver's explicit termination resistor RLOAD. The current passing through the explicit terminating resistor REX produces a voltage differential across REX that decreases across REX in the direction from terminal 4 to terminal 5.
FIG. 1B is a block diagram of the same driver circuit 1 shown in FIG. 1A. However, in FIG. 1B, the driver circuit 1 is configured with switches 7 and 9 closed and switches 6 and 8 opened. When switches 7 and 9 are closed, the current follows the paths represented by arrows 13A and 13B. The portion of the current represented by arrow 13A passes through the explicit terminating resistor REX while the portion of the current represented by arrow 13B continues along the transmission line 15 to the receiver circuit 11 and through the receiver's explicit termination resistor RLOAD. The current represented by arrow 13A that passes through the explicit terminating resistor REX produces a voltage differential across REX that decreases across REX in the direction from terminal 5 to terminal 4.
Thus, when switches 6 and 8 are closed and switches 7 and 9 are opened, the signal that drives the receiver circuit 11 is positive in polarity. Conversely, when switches 7 and 9 are closed and switches 6 and 8 are opened, the signal that drives the receiver circuit 11 is negative in polarity. By operating the driver circuit 1 in this manner, changes in the polarity of the driver circuit output signal can be used to represent binary 1s and 0s. One known signaling format of this type is called nonreturn-to-zero (NRZ).
In order to limit voltage or current reflections, the output impedance of the driver circuit 1 should be matched to the impedance of the load and transmission line. For ease of explanation, it will be assumed that the impedance of the load RLOAD is equal to the characteristic impedance of the transmission line. Therefore, it will also be assumed that the goal of impedance matching is to match the output impedance of the driver circuit 1 with the impedance of the load, which in this case is the explicit termination resistor of the receiver 11 RLOAD.
One approach to matching the output impedance of the driver circuit 1 to the impedance of the load is to set the value of the explicit resistor REX equal to RLOAD. However, setting REX equal to RLOAD would require that the current sources generate a relatively large current, which, in turn, would require that the FETs that constitute the current sources be very large in size. Increasing the size of the FETs would, in turn, result in heavily loading the output of the driver circuit 1 with parasitic capacitance, which is undesirable.
In order to keep the sizes of the current source FETs to a minimum, it is known to set the value of the terminating resistor REX equal to twice the size of the load impedance (i.e., REX=2(RLOAD)). While setting the value of REX equal to 2RLOAD does not provide perfect impedance matching, it limits the buildup of high noise levels caused by multiple reflections between the driver circuit 1 and the receiver circuit 11. Setting REX equal to 2RLOAD also limits the current passing through the terminating resistor REX, which is essentially “wasted” because it is not provided to the load.
In addition, the output impedance of the driver circuit 1 depends in part on the impedances of the current sources 2 and 3, which result from practical non-idealities of the current sources. The impedances of the current sources 2 and 3 change with changes in the operating conditions of the current sources. For example, as a current source goes into the triode region of operation, the impedance of the current source decreases. Setting the value of the terminating resistor REX equal to 2RLOAD does not take into account changes in the output impedance of the driver circuit that result from changes in the impedances of the current sources.
A need exists for a driver circuit that has an output impedance that is precisely matched to the load impedance and that is efficient in terms of power consumption. In addition, because the output impedance of the driver circuit changes as a result of the changing impedance of the current sources, a need exists for a driver circuit that has an output impedance that is set to compensate for changes in the impedance of the current sources of the driver circuit.